The polymerization of propylene and mixtures of propylene and other monomers with Ziegler Natta type catalysts is well known in the art. Such catalysts typically consist of two components, an active transition metal compound such as a titanium halide and an organoaluminum compound such as an aluminum alkyl.
Propylene, with or without other monomers, is fed into a reactor with the catalyst. Hydrogen, which acts as a chain terminator, is used to control the degree of polymerization and consequently the melt flow rate of the homo or copolymer produced during the polymerization reaction.
Normally only one reactor is required to produce propylene homopolymer and random propylene/ethylene copolymer. In some cases, however, multiple reactors are used in series to obtain specific properties. For example, U.S. Pat. Nos. 4,760,113, 4,500,682 and 4,499,247, which are incorporated herein by reference, disclose multiple stage polymerization of propylene. The present invention relates to the use of propylene-ethylene copolymers produced by multiple stage polymerization in multiple cavity stretch blow molding applications.
Stretch blow molding, also known as biaxial orientation blow molding, is a well known process for producing articles such as food and beverage containers. The stretch blow molding process generally involves the following steps: (1) forming a parison or preform by extrusion or injection molding; (2) thermally conditioning the preform; (3) optionally pre-blowing the preform either before, after or during the conditioning process; and (4) stretch blowing to the final shape. Biaxial stretch blow molding of a thermoplastic material is a preferred process for producing small to medium size containers since the process will normally increase the material's tensile strength, barrier properties, drop impact and clarity. Stretch blow molding is described in Blow Molding Handbook, pp 52-56, 117-148, Hanser Publishers, (1989); Plastics Blow Molding Handbook, pp 83-114, Van Nostrand Reinhold, (1990); and Winkle et al, Extrusion Thermoforming and Stretch Blow Molding of Polypropylene, Advances in Polymer Technology, vol. 2 No. 2 pp. 107-140 (1982), all of which are incorporated herein by reference. Commercial stretch blow molding equipment is manufactured by Johnson Controls, Manchester Mich., Aoki Manufacturing Co. Tokyo, Japan, Nissei ASB Machine, Tokyo, Japan and others. Normally machines used for stretch blow molding are multiple cavity machines, having 4, 8, 16 or 32 cavities for continuous production.
Although it is possible to use a number of thermoplastic materials such as acrylonitrile, polystyrene, polyvinyl chloride, nylon, polycarbonate, polysulfone, acetal, polyarlyate, polypropylene and surlyn in stretch blow molding applications, polyethylene terephthalate has dominated the market for stretch blow molding applications due to its amorphous nature which allows stretch blowing immediately after cooling to stretching temperature. Alternatively, semicrystalline materials such as polypropylene must be cooled until substantial formation of crystalline regions occurs as the improvement in properties is dependent upon the orientation of crystalline regions in the material. Others factors weighing against the use of semi-crystalline materials such as polypropylene in stretch blow molding processes include obtaining the necessary uniformity in material distribution throughout the container walls and the precise temperature control normally required to achieve biaxial orientation of the material.
Uniformity of material distribution is especially desired in stretch blow molding processes as nonuniform distribution requires the use of additional material to insure that all portions of the container meet functional requirements. In the case of multi-cavity machines, variation between containers produced in the different cavities is another factor contributing to nonuniformity.
An important consideration in stretch blow molding operations is the cycle time which is dependent, at least in part, on the rate at which the material crystallizes. The rate at which a semicrystalline material such as polypropylene crystallizes can be increased with the addition of an extrinsic substance which acts as a seed or nuclei on which crystal growth can be initiated. Such substances are commonly referred to as nucleation agents and may consist of inorganic substances such as talc and other silicates, precipitated or ground calcium carbonates, sodium phosphates and stearates. Organic nucleating agents include dibenzylidene sorbitols and sodium benzoate. During the melt process, these substances either do not melt at all, or melt but solidify before the polymer, thus acting as nuclei for the initiation of crystallization.
The use of conventional nucleating agents has several disadvantages. First, the efficiency of the agent depends upon its particle size and the degree of dispersion and distribution of the agent in the polymer. Inorganic nucleating agents must have an extremely small particle size and be uniformly dispersed and distributed throughout the polymer to be efficient. Moreover, the addition of any foreign substance to the polymer can affect the physical and chemical properties, such as toxicity and extractability, of any product made from the polymer. This is especially critical in the case where the polymer is used to make a product that will come in contact with food or medical product.
Preferably, crystallization enhancement is achieved by treating or using a relatively small amount of material to facilitate processing of the polymer. It is also desirable that crystallization enhancement be accomplished without degrading the polymer. The limitations of prior art nucleating agents are overcome in the practice of the present invention by utilizing a nucleating agent comprising an irradiated polypropylene resin.
European Patent Application 88308452.7 discloses the use of propylene-ethylene resins having a melt flow rate greater than 50 gm/10 min, preferably about 60 or greater for producing stretch blow molded containers. European Patent Applications 84115106.1 and 87110734.8 disclose containers with a side wall having a percentage haze of 9% or smaller when converted to a wall thickness of 1 mm obtained by injection stretch blow molding of a propylene based resin. These references do not, however, address the issues of uniform material distribution or cycle time.
The foregoing references have not overcome the limitations of the prior art with respect to the use of propylene polymers for biaxial orientation blow molding process. Consequently, there exists a need for propylene based resins with enhanced mechanical and crystalline properties for use in stretch blow molding applications that overcome the inherent limitations of prior art resins.